An insider is a shortening of the term inside joke, which means a joke that is shared between you and your friends which only makes sense to the people that were with you when whatever it was happened. In other words, it's a "had to be there moment."

Guy: OMG i went to some pet store, and there were some bunnies there. They were really cute.

Condition associated with people who stay indoors for weeks at a time leading to the formation of strange moods and extra talkative behaviours. Computer nerds, students, weed smokers and unemployed people are very prone to "insiders"

1. He hasn't been outside for weeks, he's talking rubbish and has a really bad case of insiders.